1. Field of the Invention
The present invention relates to a fluid dynamic bearing unit.
2. Description of the Related Art
Fluid dynamic bearing units support a shaft member without contact by a fluid's dynamic pressure effect occurring in their bearing clearances. Having such characteristics as high-speed rotation, high rotation accuracy, and low vibration, bearing units of this type are suitably used in motors that are mounted on various types of electric apparatuses including information devices. More specifically, they are used as spindle-motor bearing units in magnetic disk drives such as HDD, optical disc drives such as CD-ROM, CD-R/RW, and DVD-ROM/RAM, and magneto-optical disc drives such as MD and MO, and motor bearing units in polygon scanner motors of laser beam printers (LBP), color wheel motors of projectors, fan motors, and the like.
For example, a fluid dynamic bearing unit to be built in a spindle motor of a disk drive such as HDD has a radial bearing portion for supporting a shaft member in radial directions and a thrust bearing portion for supporting the shaft member in thrust directions, both of which may be configured as fluid dynamic bearings. Among the known examples of the radial bearing portion in this type of fluid dynamic bearing unit are ones in which dynamic pressure generating grooves are formed as a dynamic pressure generating portion in either one of the inner periphery of a bearing sleeve and the opposed outer periphery of a shaft member, with a radial bearing clearance between the two peripheries (for example, see Japanese Patent Application Laid-Open No. 2003-239951).
Now, information devices that incorporate fluid dynamic bearing units of the foregoing configuration, such as HDD or other disk drives, require even faster rotations for the sake of a further increase in read speed. This increases a moment load to act on the bearing portions that rotatably support the spindle shaft. To address this increased moment load, it is necessary to provide a plurality of radial bearing portions at axially separated positions, with an increased span between the radial bearing portions. In a conventional configuration, the plurality of radial bearing portions are formed on the inner side of a single bearing sleeve. Due to demands for motors of smaller sizes and for spindle shafts and bearing sleeves of accordingly smaller diameters, however, it is sometimes difficult to manufacture a bearing sleeve that is capable of an increased span between the radial bearing portions.
As means for increasing the span between the radial bearing portions and facilitating the manufacturing of the bearing sleeve as well, a plurality of bearing sleeves may be arranged in a plurality of positions axially separated from each other (for example, see the publication of Japanese Patent No. 3602707).
To arrange the bearing sleeves in a plurality of positions, each of the bearing sleeves is fixed to the inner periphery of the housing by adhesion, press fit, and the like. With adhesion, however, the fixing operation takes a lot of time and labor since the adhesion process must be carefully performed so that fluid channels for a lubricating fluid, formed in the outer peripheries of the bearing sleeves, may not be filled up with the adhesive. For press fit, sufficient fixing power is secured by increasing the interference between the outer peripheries of the bearing sleeves and the inner periphery of the housing. This narrows the inside diameter of the bearing sleeves with a decrease in the radial bearing clearances, possibly causing unfavorable effects on the radial bearing performance such as an increased torque loss. There is thus the need for improvements in terms of the fixing operation of the bearing sleeves and the provision of the fixing power.
Moreover, even when any of the foregoing fixing means including adhesion, press fit, and the like is employed, if the housing has a coefficient of linear expansion greater than that of the bearing sleeves, the bearing sleeves undergo a compressive force from the housing to shrink in the inside diameter due to a difference in thermal contraction between the members with decreasing temperature. This can produce unfavorable effects on the radial bearing performance for the same reason as mentioned above.